Addition agent for the electrolyte used in the electrodeposition of lead

ABSTRACT

THE UNIFORM ELECTROYLTIC DEPOSITIONOF LEAD FROM A CONVENTIONAL ELEXTROEFINING AQUEOUS ELECTROYLTICSOLUTION CONTAINING LEAD FLUOSILICATE AND HYDROFLUOSILICIC ACID IS PROMOTED BY ADDING TO THE ELECTROLYTIC SOLUTION FROM ABOUT 140-1200 MG./LITER OF A LIGNOSULFONATE AND FROM ABOUT 40-600 MG./LITER OF A WATER-SOLUBLE BLOCK COPOLYMER OF PROPYLENE OXIDE AND ETHYLENE OXIDE HAVING THE FOLLOWING GENERAL FORMULA:   HO-(CH2-CH2-O)Y-(CH(-CH3)-CH2-O)X-(CH2-CH2-O)Z-H   IN WHICH X IS AT LEAST 13, AND Y AND Z ARE SUCH THAT THE ETHENOXY CONTENT REPRESENTS FROM 20% TO 90% OF THE COPOLYMER.

U nited States Patent Olfice 3,554,884 Patented Jan. 12, 1971 ABSTRACT OF THE DISCLOSURE The uniform electrolytic deposition of lead from a conventional electrorefining aqueous electrolytic solution containing lead fluosilicate and hydrofluosilicic acid is promoted by adding to the electrolytic solution from about 140-1200 mg./liter of a lignosulfonate and from about 40-600 mg./ liter of a water-soluble block copolymer of propylene oxide and ethylene oxide having the following general formula:

I'IO(CIIQCHZOLKEJHCIIQO):(CH2CH2O)1H in which x is at least 13, and y and z are such that the ethenoxy content represents from 20% to 90% of the copolymer.

BACKGROUND OF THE INVENTION Impure lead bullion has been electrolytically refined by the well-known Betts process for many years. In this process, the impure lead is cast in the form of slabs which comprise the anodes of an electrolytic cell. Refined lead cathode starting sheets are interspersed with the anodes in the cell. The electrolyte comprises an aqueous solution containing, typically, from about 4 to 10 percent lead as lead fluosilicate and from about to 12 percent free hydrofluosilicic acid.

A problem associated with the electrolytic refining of lead is that of establishing and maintaining a dense, smooth cathode deposit. It is known that the lead tends to deposit on the cathode in a rough, irregular pattern with the formation of growths, known as knobs or trees. As deposition continues, these growths extend to and contact the anodes, thereby causing short circuits and reducing the efficiency of the cell.

This problem is well known, and many addition agents have been suggested and tried in an efiort to overcome it. For example, small amounts of glue, and mixtures of glue and Goulac (a crude calcium lignosulfonate byproduct of the sulfite paper process), have been employed to improve the cathode deposit and overcome the problem of irregular growths. Other addition agents for the electrolyte include extracts of western red cedar and certain aloes, either by themselves or in combination with Goulac. However, while these known addition agents have improved the character of the cathode deposit to a limited extent, the problem of irregular or knobby cathode growths has not been completely solved.

Prior investigations have shown that the lignosulfonatecontaining residue from the acid sulfite pulping of wood can be used as an additive for the Betts process electrolyte to obtain cathode deposits of improved density and uniformity. However, when used by themselves, these lignosulfonate materials do not produce cathode deposits of sufiiciently high quality to justify using them in place of the more expensive but more efl'ective glues, aloes and other addition agents now being employed in commercial processes. After an intensive investigation into the problem of improving the uniformity and quality of Betts process cathode deposits, and in particular in the use of lignosulfonates derived from spent sulfite liquors as additives in the Betts process, I have made the surprising discovery that block copolymers of propylene and ethylene oxides (which by themselves are not effective addition agents for Betts process electrolytes) unexpectedly and synergistically improve the efiectiveness of acid-soluble lignosulfonate solids for this purpose. (A typical Betts process electrolyte contains from about 4 to 10 percent by weight lead in the form of lead fluosilicate and from about 5 to 12 percent by weight free hydrofluosilicic acid.)

SUMMARY OF THE INVENTION In accordance with the aforementioned discovery, I employ, as an addition agent, a mixture of a lignosulfonate and a Water-soluble block copolymer or propylene oxide and ethylene oxide in the Betts (or similar type) process for the electrodeposition of lead. The concentration of lignosulfonate in the electrolyte solution is from about -1200 mg./liter, and the concentration of the block polymer is from about 40-600 mg./ liter. The cathode deposits obtained by the use of an electrolyte solution containing my new additive mixture are smooth, uniform and substantially free of knobs and trees. In addition, on a pound for pound basis my new additive is up to five times more effective than the additive comprising a mixture of Goulac and aloes currently employed in large commerical lead refineries, and my additive materials cost only about half as much per pound as the aforementioned Goulac and aloes additive.

DETAILED DESCRIPTION The term lignosulfonate as employed herein refers to the lignosulfonate-containing residue from the acid sulfite pulping of wood or other lignocellulose-containing materials, usually referred to as spent sulfite liquor or spent sulfite liquor solids. In the acid bisulfite pulping process, the lignocellulose plant material (usually, but not necessarily, wood) is cooked in a solution of sulfurous acid and a metal or ammonium bisulfite. The cation combined with the bisulfite ion is generally referred to as the pulping base, the pulping base cations normally employed being calcium, sodium, ammonium and magnesium. Spent sulfite pulping liquors derived from any of these pulping base cooking liquors are suitable for use in the practice of my invention. However, I presently prefer to use spent sulfite liquors from sodium or ammonium base pulping processes because of their greater solubility. Irrespective of the base, spent cooking liquors resulting from the sulfite pulping of conifer woods normally contain approximately 65 percent lignosulfonates, 25 percent wood sugars, and 10 percent inorganic acids and other impurities on a dry base. These spent sulfite liquors can be employed either in their crude form containing wood sugars as well as other impurities, or they can be employed in the form of purified, essentially sugar-free lignosulfonates prepared, for example, by the process described in US. Pat. 3,271,382. Moreover, these lignosulfonate-containing spent liquors can be employed in the form of an unconcentrated aqueous solution comprising the crude of purified spent liquor. However, the spent liquor (whether crude or purified) is advantageously first concentrated to a solids content of about 40 to 50% by Weight, and preferably the thus concentrated liquor is dried, as by spray drying, to obtain a dry, powdered product. Accordingly, the term lignosulfonate as employed herein is intended to cover both the crude and the purified, and both the liquid and the dried, spent sulfite liquor material.

The block copolymers of propylene and ethylene oxides used in the electrolyte additive of my invention can be described structurally as bispolyethenoxy polypropylene oxides. These compounds may be represented by the general formula Suitable compounds are derived from polypropylene oxides having a specific viscosity of at least 0.129 in 4% benzene solution at 25 C., which are substantially water insoluble (calculated to have an average chain length of at least 13 propenoxy groups and two terminal hydroxyl groups) by substitution of the hydrogens in both terminal hydroxyl groups by polyethylene oxide chains. These compositions consist of mixtures of compounds in which the end polyethenoxy chains, y and 1 may be of equal or unequal length, but which tend to be approximately equal in length. With a polypropenoxy chain, the minimum length corresponding to a substantially water-insoluble product as represented by a specific viscosity of 0.129 in 4% benzene solution at 25 C. or a calculated average length of 13 propenoxy groups, will in general require a content of at least ethylene oxide to render the final copolymer water soluble but a content of 40-75% ethylene oxide is preferred. Where, however, much longer polypropylene oxide chains are used as represented by polypropylene oxide with specific viscosities in a range of 0.257 to 0.584 in 4% benzene solution at C., much larger contents of ethylene oxide may be desirable, in some cases of the order of 8090%.

The addition of hydrophilic ethenoxy groups to Waterinsoluble polypropylene oxide to give ethenoxy contents as described results in water-soluble block copolymers. Particularly suitable block copolymers of propylene and ethylene oxides for use in the invention are sold commercially under the trade names Pluronic L64 and Pluronic F-68 by Wyandotte Chemicals Corporation. Pluronic L64 results from addition of ethylene oxide to a hydrophobic polypropylene oxide with molecular weight of about 1750 in amount to give a liquid, water-soluble block copolymer with ethenoxy content of about 40%. Pluronic F-68 results from addition of sufiicient ethylene at concentrations of from 140-1200 mg./liter. Best results are obtained with a concentration of lignosulfonate solids approximately 2-3.5 times that of the block polymer. (The maximum amount of addition agent used is not critical and is determined by solubility and ecomonic considerations.)

The following examples are illustrative, but not limitative, of the practice of my invention.

EXAMPLES 1-6 Experimental runs were carried out in a multiple electrolytic apparatus in which the individual cells (each being a 32 ounce jar) were positioned in a constant temperature bath and provided with holders for the lead anodes and cathodes. Each cell or jar was also provided with a veriical reciprocating stirring device, and the electrode holders were connected to a regulated supply of. direct current. The electrolyte contained 140 grams per liter (g./l.) PbSiF -2H O and 92 g./l. H SiF Each cell was provided with two lead anodes positioned one on each side of a single lead cathode, the anodes being spaced one inch from the cathode. The plated area on the lead cathode was approximately five square inches, and the cathode current density was maintained at approximately 17 amps/sq. ft. Electrolyte deposition of lead was carried out for hours at 30 C. in each case.

The following additives were employed in the electrolytic solution:

Name: Description SBS-26l Sodium base spent sulfite liquor SOlids (s ray dried without neutralization).

Pluronics F-68 A block copolymer prepared by the addition of ethylene oxide to the two hydroxyl groups of a polypropylene glycol nucleus. 'F68 is a solid product with a molecular weight of 8350.

The experimental conditions and description of the electro-deposited lead are shown in the following table.

* Deposition continued 7 days at 40 C.

oxide to a hydrophobic polypropylene oxide base with molecular weight of about 1750, to give an ethenoxy content in the final solid, water-soluble copolymer of about 80%.

The discovery of the advantages for conjoint use of the block copolymers and lignosulfonates as additives in electrorefining baths was surprising and unexpected. The block ploymers alone produce voluminous sponge lead cathode deposits totally unsuitable for the electrorefining of lead. Lignosulfonates alone produce a relatively smooth plate, but with heavy edges. However, conjoint use of the block polymers and lignosulfonates has the effect of smoothing the edges and in general of producing a much more uniform plate, essentially free of nodules and trees. Generally, in the practice of the invention the block copolymers are used in the fluosilicate lead baths at con centrations of 40-600 mg./liter along with lignosulfonates o Additive:

EXAMPLES 7 AND 8 Experimental runs 7 and 8 were made in the same apparatus and using the same electrolyte and method as in the preceding examples. After 40 hours deposition at 30 C., the following results were obtained:

The experimental conditions and description of the electrodeposited lead are shown in the following table:

TABLE 2 Weight deposited, Experiment Additive 1ng./]. glns. Description Rating 7 "$25 89.9 S1110oth Good. 8 Egg 92.0 Mao Do.

polymer of polypropylene oxide and ethylene oxide having the general formula:

HO(OHzCHzOhXfiJHCHzO):(CHzCHzO) H in which x is at least 13, and y and z are such that the ethenoxy content represents from 20% to 90% of the copolymer.

2. The process of claim 1 in which the ratio of lignosulfonate to block copolymer is about 2-3.5:1 by dry weight.

3. The process of claim 1 in which the ethenoxy content represents from 4075% of the copolymer.

References Cited UNITED STATES PATENTS 2,877,165 3/1959 Turner 204-53 2,086,841 7/1937 Bagley et al. 20453 1,464,506 8/1923 Groff 204-53 JOHN H. MACK, Primary Examiner R, L. ANDREWS, Assistant Examiner 

